Active matrix type liquid crystal displays provided with a switching element for each pixel have recently been researched and developed at many institutes for use as thin picture displays, especially for liquid crystal matrix displays. A TFT of the MIS type is generally used as the switching element.
FIG. 4 schematically shows an example of the structure of an active matrix type liquid crystal display which uses a TFT. Reference numeral 21 represents a TFT, and 22 a liquid crystal layer for one pixel which is clamped between a pixel electrode (not shown) connected to the drain electrode of the TFT 21 and a counter electrode 23 provided on a substrate which is on the opposite side of the liquid crystal layer as the substrate provided with the TFT. Reference numeral 2 represents a gate wiring for connecting the gate electrode of each TFT 21 and for supplying a scanning signal which turns each TFT 21 on and off for each line to the gate electrode of the TFT 21. The reference numeral 6 denotes a source wiring for connecting the source electrode of each TFT 21 and supplying a picture signal to the source electrode of each TFT 21 which is selected by the gate wiring 2. The principle of display using an active matrix type liquid crystal display will now be briefly explained with reference to FIG. 4. For example, when a select signal is applied to a signal terminal Xi in the gate wiring 2, all TFTs 21-a which are connected to the terminal Xi are turned on at once, and a picture signal is supplied from the signal terminal Yi, Yi+1, . . . of each source wiring 6 to the pixel electrode which is connected to the drain electrode through the source electrode of each TFT 21-a. The voltage of the pixel electrode and the voltage of the counter electrode 23 determine the voltages applied to the respective liquid crystal layers 22, and the determined voltages change the light transmittances of the respective liquid crystal layers to effect the display. When the signal applied to the signal terminal Xi assumes a non-selected state and each TFT connected to the terminal Xi is turned off, a select signal is applied to the subsequent signal terminal Xi+1, and the above operation is repeated. The voltage applied to each liquid crystal layer 22 is retained due to the capacity component of the liquid crystal layer 22 itself even after the TFT 21 is turned off, until the respective TFT is turned on again.
A reverse staggered type a-Si TFT in which a gate electrode is disposed on the layer under a gate insulation layer and a semiconductor layer and a source electrode and a drain electrode are disposed on the layer above the gate insulation layer and the semiconductor layer is widely utilized as the TFT 21. A reverse staggered a-Si TFT having the structure shown in FIGS. 5 and 6, in which a gate insulation layer, an amorphous silicon layer and a protective insulation layer are subsequently formed in that order, has been proposed from the point of view of reliability and reproducibility.
In FIGS. 5 and 6, reference numeral 1 represents an insulating substrate, 2 a gate wiring, 3 a gate insulation layer, 4 an amorphous silicon layer, 5 a protective insulation layer, 6 a source wiring, 7 a gate electrode, 8 a source electrode, 9 a drain electrode and 10 a pixel electrode.
The number of pixels in an active matrix type liquid crystal display is between several thousand and several hundred thousand, and the number of intersections of the gate wirings and the source wirings is about the same. If even one intersection has a defective insulation such as shorted insulation or defective conduction such as a cut conductor, a line defect occurs on the display screen. This defect is not allowed in a finished product. Therefore, when an a-Si TFT having the structure shown in FIG. 6 is used, an interlayer insulation layer consisting of a gate insulation layer, an amorphous silicon layer and a protective insulation layer, as shown in FIG. 5, is conventionally provided at the intersection of the gate wiring 2 and the source wiring 6. FIG. 7 shows the structure of this portion in which the gate insulation layer 3, the amorphous silicon layer 4 and the protective insulation layer 5 are provided between the gate wiring 2 and the source wiring 6 as the interlayer insulation layer. It has already been proved that such structure is very effective as a measure to counter defective insulation.
In the above-described structure, there is a possibility of producing a disconnection in the source wiring 6 at the end portions of the amorphous silicon layer 4 and the protective insulation layer 5 due to a difference in level. Especially when a commonly used ITO thin film is used for the source wiring 6, since the ITO film formed has a columnar structure, there is a strong probability of producing interstage disconnection or defective conduction at the above-described end portions. If the source wiring 6 is disconnected the picture signal is not effective beyond the position at which the disconnection is produced, resulting in a line defect on the display screen. Such a defect is not allowed in a finished product. Thus, disconnections of the source wiring at the above-described portions great lower the production yield of the products of the prior art.